1. Field of the Invention
The invention relates to an operating method for an implantable cardiologic device, in particular a heart pacemaker, for detection of a significant event or condition a patient is exposed to, such as a change from day to night, a pathologic condition or the like.
2. Background Art
As for the background of the invention, clinical studies in connection with heart pacemakers examined the unipolar intracardiac impedance for possible conclusions to be drawn from the measured results as to a possible improvement of the possibilities of control of heart pacemakers. Subject of these examinations was a heart pacemaker which transmitted corresponding impedance data to an external monitor. On the occasion of two after-care examinations taking place six and twelve weeks after implantation of the pacemaker, data on the signal course of the measured intracardiac impedance were detected and stored over a period of 24 hours. From this measurement of the intracardiac impedance, the morphologic signal course of the impedance was detected so that the impedance curve of the heart during a heartbeat could be recorded. As explained below in conjunction with the exemplary embodiment, collective features and differences of the measured impedance curves were analyzed by statistical methods during the two after-care examinations. The idea was to make use of the results obtained for further optimization of the heart pacemaker functions, for instance in connection with day/night adaptation of the control parameters or for recognition of pathologic irregularities such as susceptibility to extrasystoles or sudden cardiac death.
The clinical studies have for example shown that the morphology of the averaged unipolar intracardiac impedance is comparatively stable for the period of the after-care examinations. Statistical evaluations have found differences between various patients in the correlation of the measured values; these differences can be exploited for diagnostic purposes. Finally, differences between the average day and night curves offer a criterion for simple day/night differentiation and recognition.
Proceeding from this, the invention proposes an operating method for an implantable cardiologic device, in particular for a heart pacemaker, having certain method steps for detection of a significant event or condition a patient is exposed to. In this regard, the invention does not involve complete control of a heart pacemaker, but the partial aspect defined in the purpose specified. Within the scope of a certain control program being implemented on a heart pacemaker, a corresponding function can be allotted to this partial aspect, this function being fulfilled by the operating method according to the invention.
This operating method comprises the following method steps:
measurement of intracardiac impedance;
detection of the morphologic signal course of the impedance based on at least one morphologically representative parameter of the signal course;
continuous storage of the parameter of a defined measuring period;
computation of a correlation coefficient for the stored parameter; and
comparison of the correlation coefficient with a reference correlation coefficient, with transgression of a defined deviation of the correlation-coefficient from a reference correlation coefficient indicating the presence of the significant event.
The method bases on the measurement of the intracardiac impedance of the heart and detects the morphologic signal course of the impedance by an as a rule unipolar electrode that is anchored in the myocardium, which means measurement not only of a certain value, but of the impedance in dependence on various instants during a cycle of cardiac contraction. In the extreme, one parameter and preferably two morphologically representative parameter data of the signal course, such as the peak-to-peak amplitude can be sufficient as parameter data. These parameter data are then detected over a defined measuring period which may take several hours of the day, depending on the storing capacity of the implantable cardiologic device. Correlation coefficients are computed for these stored parameter data, to which end varying computing methods may be used, depending on the function and design of the operating method. In this regard, xe2x80x9ccorrelation coefficientxe2x80x9d does not exclusively mean the computation of a correlation along strictly statistical lines. It is to imply generally the determination of expressive values such as the computation of the square deviation, standard deviation, autocorrelation, the virtual correlation strictly speaking, the cross correlation, or even simply the deviation of a signal course from a standard or reference signal course. This will be explained in detail below.
For the purposes of the invention, the correlation coefficients are compared with a reference correlation coefficient, the presence of a significant event being indicated whenever a defined deviation between these two correlation coefficients is exceeded.
In accordance with further preferred embodiments of the method, the various correlation coefficients can be continuously computed and compared with the reference correlation coefficients. This means continuous monitoring of the heart for any possible deviations from standard, it being possible, for evaluation purposes, continuously to store preferably the correlation coefficients computed during a certain interval. On the whole, what takes place is an evaluation of the parameter data of the signal course, accessing a certain window of time which, during operation of the implantable cardiologic device, extends backwards from the respective current time by the duration of the evaluation interval.
During operation of the implantable cardiologic device, the mentioned evaluation methods may for instance be employed to recognize a change from day to night or from night to day. If for instance autocorrelation coefficients are continuously computed and evaluated by comparison of parameter data that differ by a defined time lag, a change from day to night (or vice versa) will occasion a decrease of the autocorrelation coefficient as a result of the accompanying change of the typical signal course; after passage of the change through the interval, the autocorrelation coefficient rises again to its initial value. The change generates a minimum in the chronological order of the autocorrelation coefficients, which is recognized and may be used for instance for adjustment of the operating program of the implantable cardiologic device from day operation to night operation.
According to another preferred embodiment of the operating method according to the invention, any possible pathologic conditions of the heart can be recognized by the determined correlation coefficients being analyzed and by the transgression of a defined deviation from reference correlation coefficients being recognized as the presence of a possibly pathologic condition. Correspondingly, the implantable cardiologic device may set a warning signal which is detectable from outside.
Since competent clinical examinations have shown that the correlation coefficients may change significantly in dependence on the time for which the implant has been implanted, it is of advantage to make the reference correlation coefficients adjustable in dependence on the duration of implantation of the implant. Thus, the operating method that takes place in the implanted device is optimally adjusted to the action time thereof.
A special embodiment of the operating method according to the invention aims at the statistic and spectral analysis of the parameter data of the signal course of the average intracardiac impedance. As a correlation coefficient for the stored parameters, it will do to detect the deviation of the individual signal coursesxe2x80x94for instance the square or standard deviationxe2x80x94from an averaged signal course or from each other and to analyze same by means of spectral analysis. This analysis proceeds from detecting deviations of comparatively small frequencies that repeat themselves regularly and are coupled with physiologic functions of the body in a manner still to be explained. Based on this detection, the operating method may implement corresponding actions in the cardiac pacemaker, which adjust themselves to the detected situation.
According to another preferred embodiment, the operating method according to the invention can be designed for the possibility of selection between varying evaluation methods and parameters as indicated in the foregoing claims. This selection may take place automatically in dependence on the preceding result of evaluation by corresponding implementation of the operating program. Even a purely preset selection by the operating program is conceivable without any reference to the prior results of evaluation.
The following is a detailed explanation of the scientific findings underlying the invention and the operating method based thereon concerning a cardiac pacemaker in its varying forms, taken in conjunction with the drawing.